Throughout this application various publications are referred to in brackets. Citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications are hereby incorporated by reference in their entireties into the subject application to more fully describe the art to which the subject application pertains.
The term “melanin” originates from a Greek word for black “melanos.” Melanin is a high molecular weight pigment, ubiquitous in nature, with a variety of biological functions [1]. Melanins are found in all biological kingdoms. These pigments are among the most stable, insoluble, and resistant of biological materials [2]. Melanins can have different structures depending on the biosynthetic pathway and precursor molecules. Some definitions of melanin have focused on chemical and physical properties of melanins instead of defined structures [46]. Melanins can be synthesized in the laboratory by chemical means or by many living organisms. Melanins formed by the oxidative polymerization of phenolic compounds are usually dark brown or black [2]. However, melanins may have other colors as illustrated by the finding that dopamine-derived melanin is reddish-brown. Fungi can make melanins from at least two major biosynthetic pathways, employing the precursor 1,8-dihydroxynapthalene (DHN melanin) or the oxidation of suitable tyrosine derivatives like dihydroxyphenylalanine (DOPA-melanin) [2]. The fungus C. neoformans can make melanins from a wide variety of phenolic compounds which are oxidized by a laccase enzyme [47-49].
Many fungi constitutively synthesize melanin [2], which is likely to confer a survival advantage in the environment [3] by protecting against UV and solar radiation [reviewed in 4]. Melanized microorganisms inhabit some remarkably extreme environments including highland, Arctic and Antarctic regions [5]. Most dramatically, melanized fungal species colonize the walls in the high constant radiation field of the damaged reactor at Chernobyl [6] as well as the soils around the damaged reactor [7]. These findings and laboratory observations of the resistance of melanized fungi to ionizing radiation [8,9] suggest a role for this pigment in radioresistance.
Despite the presence of melanotic microorganisms in radioactive environments it is unlikely that melanin is synthesized solely for the purposes of protection (shielding) from ionizing radiation. For example, in high elevation regions inhabited by melanotic fungi the background radiations levels are approximately 500-1,000 higher than at sea level, which amounts to a dose of 0.50-1.0 Gy/year. Since the overwhelming majority of fungi, melanized or not, can withstand doses up to 1.7×104 Gy [9], there is no apparent requirement for melanin as a protector. On the other hand, biological pigments play a major role in photosynthesis by converting the energy of light into chemical energy. Chlorophylls and carotenoids absorb light of certain wavelengths and help convert photonic energy into chemical energy during photosynthesis.
Given that melanins can absorb visible and UV light of all wavelengths [10], the inventors hypothesized that melanized microorganisms could use this pigment to scatter/absorb ionizing radiation, a phenomenon that could protect the cell from excessive radiation and also capture energy for a biosynthetic process analogous to photosynthesis. In this scenario, melanin pigment would serve as a transducer of ionizing radiation energy into chemical energy. As disclosed in the present application, melanin provides some protection of fungal cells from very high doses of ionizing radiation and serves as an energy transducer in a metabolic process termed here as “radiosynthesis.”